1. Field of the Invention
The present invention relates to a navigation system, and more particularly to a navigation system incorporating a global positioning system (GPS) receiver and a self-operating sensor for determining the position of a reception point or a mobile object such as an automobile in a global geometrical region, the navigation system having a means for correcting an error of positional data.
2. Description of the Prior Art
There are known navigation systems incorporating global positioning systems (GPS) for navigating various mobile objects such as automobiles, airplanes, ships, or the like in a global geometrical region. Typically, such a navigation system on a mobile object has a GPS receiver which receives radio waves transmitted from three or more GPS satellites, determines the position of a reception point where its radio-wave receiver has received the radio waves, based on quasi-distance data between the GPS satellites and the reception point, including a time offset of the receiver, and positional data of the GPS satellites, and outputs the data about the determined position. Since, however, the radio waves from the GPS satellites may not necessarily be received under good conditions because of receiving environments and weather conditions, some navigation systems which incorporate a GPS receiver are also combined with a self-operating sensor which produces the positional data of its own for more accurate navigation. The self-operating sensor for use in automobile navigation systems may comprise an orientation sensor such as a geomagnetic sensor or a gas-rate gyroscope, or a speed sensor such as a sensor for detecting the rotational speed of a crankshaft.
One known navigation system comprising a GPS receiver and a self-operating sensor is disclosed in Japanese laid-open patent publications Nos. 63-247613 and 63-223585, for example.
The navigation system disclosed in the former publication selects GPS data from the GPS receiver when the position determined based on the GPS positional data is located within an error range of the navigation process of the self-operating sensor, and selects the positional data from the self-operating sensor when the position determined based on the GPS positional data is located outside the error range of the navigation process of the self-operating sensor. However, if the difference between the GPS data from the GPS receiver and the positional data from the self-operating sensor is gradually increased, then the mobile object will eventually be navigated only on the basis of the positional data from the self-operating sensor, with the GPS data being not used at all. When only the positional data from the self-operating sensor is used, the navigation system suffers the disadvantage of the self-operating sensor, i.e., an accumulation of errors due to an increase in the distance by which the mobile object has traveled or in the time in which it has traveled.
According to the navigation system disclosed in the latter publication, previous and present GPS data from the GPS receiver are compared with each other, and when present GPS data that is estimated from the previous GPS data deviates from the actual present GPS data beyond a predetermined range, the actual present GPS data is ignored, and positional data from the self-operating sensor is selected for navigation. This navigation system also has its own drawbacks. Specifically, it may possibly produce an error of several hundreds meters if intentional error information known as selective availability (SA) is contained in positional data from the GPS satellites. Even if the positional data error lies within a predetermined range allowing the GPS data to be used for navigation, a path of movement of the mobile object, as determined by the GPS data, is displayed on a display unit as a plurality of short discrete line segments, but not as a continuous line. Consequently, the produced positional data tends to be inaccurate, and unable to be matched to map data.
Furthermore, in the event that a different combination of GPS satellites is used for producing GPS data, or a positioning mode switches between two- and three-dimensional positioning modes, or the ratio waves from the GPS satellites suffer the multipath effect, the path of movement of the mobile object displayed on the display unit suddenly shifts over an interval which corresponds to an actual distance ranging from several tens to hundreds meters.